WEDNESDAY, June 21, 2017 (HealthDay News) -- Women who breast-feed their babies may have a slightly lower risk of suffering a heart attack or stroke decades later, a large new study suggests.

Researchers found that among nearly 290,000 women in China, those who breast-fed were 10 percent less likely to suffer a heart attack or stroke later in life, versus women who bottle-fed their babies.

How might breast-feeding help heart health?

One theory holds that breast-feeding helps "reset" a woman's metabolism after pregnancy, according to lead researcher Sanne Peters. She's a research fellow in epidemiology at the University of Oxford in England.

The study pointed out that women who breast-feed for a longer time tend to have lower odds of high blood pressure and diabetes, for example.

Dr. Nieca Goldberg, is a spokesperson for the American Heart Association. She said it's possible that women who breast-feed simply have healthier habits in general.

In studies like the current one, researchers do try to account for those other habits. But, Goldberg noted, it's difficult to account for all the factors that come up between the time a woman breast-feeds and the point where she eventually develops heart trouble -- which may be decades later.

On the other hand, breast-feeding could have direct effects.

As an example, Goldberg pointed to oxytocin, a hormone released during breast-feeding.

"Oxytocin causes blood vessels to relax," Goldberg said -- though, she added, it's not clear whether that would affect a woman's heart health years down the road.

It's also not clear from this study's findings if breast-feeding directly curbed women's cardiovascular risks.

"But [the findings] do add to evidence that it's protective against heart disease," said Goldberg, who is also medical director of the Women's Heart Program at NYU Langone Medical Center in New York City.

The American Academy of Pediatrics recommends breast-feeding exclusively for the first six months of life, and then continuing to breast-feed while gradually adding solid foods during the next six months. After that, the decision to continue breast-feeding is up to mom and baby, the AAP says.

Most of the women in the current study -- 97 percent -- breast-fed their children for some period of time. The average time for breast-feeding was 12 months per child, the study said.

The women in the study were all free of heart problems when the study began. They were 51 years old, on average. Almost all had given birth.

Over the next eight years, nearly 17,000 women developed coronary heart disease -- which includes heart attacks and clogged arteries that can lead to a heart attack. Another 24,000 women suffered a stroke.

It also appeared that the risk kept dipping the longer a woman had breast-fed. For each additional six months of breast-feeding per baby, the odds of cardiovascular trouble declined by 3 to 4 percent, on average.

Of course, women who breast-feed and those who bottle-feed can differ in many ways, according to Peters.

The study did account for many of those differences, she said -- including age, smoking habits, exercise levels and current body weight.

And breast-feeding was still tied to lower odds of cardiovascular trouble.

In Western countries, Peters noted, women who breast-feed are often more educated and have higher incomes. And that can make it trickier to weed out any health benefits from breast-feeding itself.

But in China, Peters said, the reverse is true: Poorer women in rural areas generally breast-fed for a longer time than their urban, higher-income counterparts.

So would these findings necessarily translate to other countries? It's hard to say, according to Goldberg.

But some studies in other countries have had similar findings, Peters said. A study of U.S. nurses, for example, found that those who'd breast-fed for at least two years over their lifetime had a lower heart disease risk.

Goldberg stressed another point: There are plenty of ways for women to protect themselves from heart disease and stroke.

"Certainly," she said, "they can lower their risks by getting regular exercise, not smoking, managing stress and eating a healthy diet -- like the Mediterranean diet."

Breast-feeding, Goldberg said, could be one more choice to put on that list.

The study was published online June 21 in the Journal of the American Heart Association.